1. Field of the Invention
The invention relates generally to a spray washer for a disc filter and more particularly to a washer that washes the slurry layer on the media of a disc filter.
2. Prior Art
Disc filters for separating solid particles from liquid are known in the art. Published International Patent application WO 93/23140 discloses a conventional rotatable disc filter. As shown in FIG. 1, the disc filter includes a plurality of hollow discs 2 having side walls 3 of a filter material. The hollow discs 2 are coupled to a hollow axle 4 through holes 9. The discs 2 are positioned within a container 5 which has an inlet 6 (shown in FIG. 2) for introducing a slurry of liquid containing solid particles into the container 5. A vacuum pump 7 draws the liquid through the discs 2. The solid particles accumulate on the surface of the filter material 3, thereby separating the liquid from the solid particles.
As shown in FIG. 2, the container 5 is filled approximately half way with the slurry. Sections of the disc 2 rotate counter-clockwise, enter the slurry and emerge coated with solid material. A scraper 11 is used to remove solid material formed on the disc 2. It is desirable to leave a certain amount of the solid material, referred to as the precoat, on the disc 2. The precoat acts as an additional filter. The scraper 11 removes a layer of solid material (referred to as the cake layer) formed on top of the precoat layer The cake falls into a chute 12 and is transferred away from the filter using known conveyance mechanisms.
After the filter has been running for some time, the precoat layer becomes less permeable and does not provide adequate filtering. Accordingly, it is necessary to periodically remove the precoat layer. The disc filter shown in FIG. 2 accomplishes removal of the precoat layer by using an oscillating spray pipe 14 fitted with a spray nozzle 15. The spray nozzle 15 moves between the periphery and center of the disc 2, sprays water on the disc 2, and removes the precoat layer.
The use of an oscillating shower to remove solid material from a disc filter is also described in U.S. Pat. Nos. 3,252,577 and 4,332,680. A problem common to all these devices is that the shower arm moves at a fixed speed. Because there is much less disc surface area at the interior of the disc, more water than necessary is applied to the inner portion of the disc which wastes water. If the speed of the oscillating shower is increased to correspond to the interior region of the disc, then insufficient spray is achieved at the outer region of the disc.
The disc filter shown in FIGS. 1 and 2 may also be equipped with an agitator as shown in FIG. 3. In order for the disc filter to operate efficiently, the solid particles must be distributed throughout the liquid. This allows the solid particles to contact the entire surface area of the disc 2 and provides for optimal filtering. In addition, the agitation prevents the solid particles from accumulating at the bottom of the container 5 and forming a solid mass. As shown in FIG. 3, an agitator 24 is placed in the container 5. Air is forced through the agitator 24 in order to agitate the slurry. However, using air as the source of the agitation has drawbacks. The liquid in the disc filter is forced towards the discs 2 through pressure in the container 5. As the liquid passes through the disc there is a pressure drop. Air bubbles entrained in the liquid will expand at this point and as a result will partially block the passage of the liquid in the precoat, and thereby reduce the filter capacity.